The present invention generally relates to semiconductor wafer processing, and more particularly relates to a method and apparatus for uniformly diffusing an impurity into a plurality of wafers.
One step in the processing of semiconductor wafers includes the introduction of impurities into the device wafer. A widely used method for introducing impurities is the diffusion annealing process.
The diffusion annealing process may be performed either in an open tube or a closed tube. In a known open tube diffusion process, the wafers are placed in a holder vertically or perpendicular to the longitudinal axis of the tube, which is generally placed horizontally. The tube and holder are placed within a diffusion furnace into which impurities are introduced in a gas stream.
An open tube diffusion process is generally used for silicon wafers. On the other hand, in the case of compound semiconductors, such as GaAs or InP, which are generally referred to as III-V compounds, surface damage such as pitting may occur to the wafers from the flow of the gaseous impurity sources. This surface damage is due to the more volatile constituent of the compound semiconductor, usually the non metal atom preferentially leaving the surface of the wafers at the elevated temperatures used in diffusion process. Such loss may be exacerbated by a flowing gaseous stream of impurities and may lead to various degrees of surface damage in an open tube diffusion anneal. Moreover, some of the impurities commonly used in III-V semiconductor devices, such as cadmium, are toxic and are not easily adapted for open tube diffusion processing in a safe manner.
In a closed tube diffusion annealing process, wafers are placed within an ampoule along with a solid diffusion source that includes impurities. The ampoule, or tube, is vacuum sealed at both of its ends. Then, the sealed ampoule is placed in a furnace and the temperature is raised to establish a solid-vapor equilibrium to drive the impurities from the vapor phase into the wafers. To prevent surface damage or pitting, it is common practice to include along with the impurity diffusion source a suitable source of the volatile element as well.
After the diffusion annealing process is completed, the sealed ampoule is removed from the furnace and cut open. Conventionally, a circular saw is used to cut around the circumference of the ampoule to recover the diffused wafers. Specifically, the ampoule is rotated on its own axis and moved forward toward the rotating blade of the circular saw.
A disadvantage of known closed tube diffusion processes is that the necessary cutting step to open the ampoule tends to break the wafers. Wafer holders used in the open tube diffusion process are not applicable to the closed tube diffusion process since such holders are not designed to hold wafers securely during rotation of the ampoule which is necessary for cutting open the sealed ampoule in the closed tube diffusion process. While vertical slot holders may be used with appropriately larger ampoules and larger furnace tubes, they are not easily adapted for restricted ampoule sizes with smaller diameter furnace tube designs, which are generally used based on thermal considerations. In such cases, accommodating several wafers vertically leads to the problem of restricted flow of the vapor phase and hence the impurity density available in the vapor phase for diffusion into multiple wafers is not uniform.
There is, therefore, a need for a wafer holder for use in closed tube diffusion annealing processes which alleviates at least to some extent the aforementioned disadvantages.
The present invention provides a wafer holder that includes a hollow holder body having an upper portion and a lower portion and a longitudinal axis extending between a first and a second end. The holder body includes a plurality of first slots and a plurality of second slots. At least the first slots slope from the upper portion to the lower portion toward the first end and are angled relative to the longitudinal axis.
The present invention further provides a diffusion annealing apparatus which includes a hollow ampoule having a pair of ends and a wafer holder including a hollow holder body having a longitudinal axis. The ampoule is adapted to be sealed at each of the ends. The holder body has an upper portion and a lower portion and a longitudinal axis extending between a first and a second end. The holder body includes a plurality of first slots and a plurality of second slots, at least the first slots sloping from the upper portion to the lower portion toward the first end thus being angled relative to the longitudinal axis.
The present invention further provides a diffusion annealing system. The system includes a diffusion source comprising an impurity, a heat source, and an annealing apparatus. The diffusion apparatus includes a hollow ampoule having a pair of ends, and a wafer holder positioned within the ampoule. Each of the ends is sealed. The wafer holder includes a hollow holder body having a longitudinal axis. The holder body has an upper portion and a lower portion and a longitudinal axis extending between a first and a second end. The holder body also includes a plurality of first slots and a plurality of second slots, at least the first slots sloping from the upper portion to the lower portion toward the first end thus being angled relative to the longitudinal axis.
The present invention further provides a method of diffusion annealing a plurality of wafers. The method includes heating a sealed ampoule housing a diffusion source having an impurity in a first region and a wafer holder holding a plurality of pairs of wafers each in a first slot in a second region and diffusing the impurity throughout the ampoule. The first slots are again angled relative to a longitudinal axis of the ampoule.
The foregoing and other advantages and features of the invention will be more readily understood from the following detailed description of preferred embodiments, which is provided in connection with the accompanying drawings.